The present invention relates to an immersed type photovoltaic detector sensitive to infrared radiation, comprising a substrate transparent to the infrared radiations; on one side of the substrate, a wafer of semiconducting material of a first type of conductivity; in this wafer, on the side opposite the substrate, at least one zone of a second type of conductivity forming, with the material of the first type, a PN junction; the substrate on the side opposite the wafer being machined in the form of a lens.
A detector of this type is already known, particularly by Applicants' French Patent Application No. 79.29470.
Such detectors are being produced at the present time with a substrate in CdTe and a wafer in Hg.sub.1-x Cd.sub.x Te, which is an alloy of CdTe, a material with a wide forbidden band, and of HgTe, a semi-metal meterial, and of which the wave-length of detection depends on the value of x. The detectivity of these detectors, defined by the signal-to-noise ratio and which is representative of their performances, is limited, in certain cases, only by their thermal noise or their capacitive noise, without being influenced by the ambient background and its fluctuations. These infrared photodetectors not limited by the ambient background are called non-BLIPs (Background limited infrared photodetectors). They are used in particular in the military field for the evaluation of lenses or for telemetry. To increase the level of the signal whilst maintaining that of the noise constant is the same as increasing the detectivity. This is what is obtained by optical immersion, by increasing the apparent surface of the detector proper.
This apparent surface may be multiplied by n.sup.2, n representating the index of the lens, if the detector is placed at the centre of a hemispherical lens, or by n.sup.4, if the detector is placed at the first Weierstrass point of a hyperhemispherical lens, extending beyond the centre of the corresponding sphere.
As to the detectivity itself, it is multiplied by n in case of hemispherical immersion and by n.sup.2 in case of hyperhemispherical immersion, if it is limited only by the thermal noise, and by n.sup.2 in case of hemispherical immersion and by n.sup.4 in case of hyperhemispherical immersion, if it is limited only by the capacitive noise.
Finally, optical immersion makes it possible to make detectors operating at intermediate temperature or at high electrical frequency, with performances higher than those of the non-immersed detectors, under the same conditions.
Two processes are known at present for making these immersed detectors.
In a first process, a detector made elsewhere is purely and simply glued on a lens.
In the second process, interdiffusion is effected of a wafer of the material of the first type of conductivity evoked hereinabove and of the substrate of the lens, after having thinned the wafer in question, in which the detector is then made, and before machining the substrate in the form of a lens. It should be noted here that, with such a process, only a detector with illumination by the rear face can be produced.
The first process, employing glue, presents several drawbacks. The glue, which constitutes an intermediate medium of low refraction index, limits, by total reflexion, the field of view of the detector. In addition, it is not easy to choose a glue which satisfies, simultaneously, the requirements of index, as has just been mentioned, of infrared radiation transmission, the coefficients of thermal expansion, mechanical solidity, rate of degassing, etc.
The second process, by interdiffusion, also poses a problem, even if it presents the advantage of eliminating the "index jump" provoked, in the first case, by the glue. In fact, the interdiffusion requires treatment at high temperatures, generally of the order of 600.degree. C., which makes it possible to produce a mosaic of diodes which are all equally satisfactory.
It is therefore an object of the present invention to propose a detector of the type mentioned hereinabove but not presenting the drawbacks of those produced by the heretofore known processes.